ARTICLE 4: LAGUNA DE ARENAL SHORELINE SURVEY

Comments

Transcription

ARTICLE 4: LAGUNA DE ARENAL SHORELINE SURVEY

ARTICLE 4:
LAGUNA DE ARENAL
SHORELINE SURVEY
M.rilynn Mueller
ABSTRACT
A survey of the Laf/Una,de Arenal shoreline in northwestern Costa Rico was conducted duri!J9
March and April of the 1984 Proyecto Prehist6rico Arenal field season. A total of 22 were located,
'which range chronologically from early Period IV Tronadora Phose (1000 - 500 B.C.) to late Period
VI Tlla;6n Phose (TOO - 1500 A.D.). Occupation appears to have been greate~t during late Period
IV Arenal Phase (500 S.c. - A.D. SOD). Environmental viariability and regional stratigraphic sequences
a" related to settlement patterns.
Deportment of Anthropology
University of Colorado
INTRODUCTION
The focus of Proyecto Prehistorico Arenal is the reconstruction of prehistoric
subsistence strategies and settlement patterns in a volcanically active area, within the
theoretical framework of human ecology. 'Thus, a major emphasis of the survey was the
location of habitation sites and other activity areas. The shoreline survey offered unique
opportunities; The shoreline crosscuts a number of different ecological zones, allowing us
to examine multiple environmental variables in relationship to settlement patterns and
potential resources. Erosion of the shoreline provided the advantage of exposure of sites
in deeper stratigraphic levels; such sites would otherwise have been difficult to detect
because they are generally buried by 1 to 3 meters of tephra deposits and soils. Another
major objective of the survey was to further define the regional stratigraphic sequence
(see Appendix A, 'Article 3). Knowledge of the regional stratigraphic sequence should
facilitate understanding of the relationships between sites, settlement patterns and
environmental variables in a chronological context.
THE SURVEY AREA
The Cuenca de Arenal is a tectonic depression between the Cordillera de Guanacaste and the Cordillera de Tilaran in northwestern Costa Rica. Today the cuenca is occupied by the laguna de Arenal, a large northwest-southeast trending lake, whose waters
at the time of the survey were at an elevation of 540 meters above sea level. The current
lake level is the result of the construction of the Sangregado earthfill dam and filling of
the lake for the Arenal Hydroelectric Project. The lake level before the construction and
filling was 512 meters; however, there is evidence of considerable fluctuations in the lake
level in the past, ranging from 480 to 530 meters in elevation (Tosi 1980). Unfortunately, at present there is little information available as to the take level at any given ti'me.,
VINCUlOS 10(1-2): 61-73,1984
Vokan Arenal and Cerro Chato, at the eastern end of the lake, dramatically domi
nate the region, both visually and geologically. Volcan Arenal is an almost perfectl
conical stratovolcano, rising to an elevation of 1633 meters, with a roughly circular base 4
kilometers in diameter. Cerro Chato southeast of Arenal is the collapsed caldera of an 01.
der volcano. These volcanoes are probably the. youngest members of a regional volcanic
cordillera running from Voldn Orosi in the northwest to Volcan Irazu in the southeast.
Volcan Arenal continues active today following its last violent eruptive phase in
1968 (Melson, in press), and must certainly have played an important role in prehistoric
adaptations in the survey area. Nine previous prehistoric eruptions are known, and the
beginning of Arenal's activity may date to 3000 S.c. (Melson, in press). The regional
stratigraphic sequence is derived from tephra deposits from eruptions of Arenal and possi·
bly Cerro Chato as well (Article 3).
Underlying these more recent volcanic strata is the Aguacate Formation which
consists of a thick series of agglomerates, andesite and basalt lava flows, breccias, and
tuffs of the Miocene to Pliocene eras (Castillo-Munoz 1983). This formation is readily
recognizable in the field as an orange-to-red weathered formation of extraordinarily high
clay content. The overlying more recent volcanic deposits at the western end of the lake
may measure as little as 20 centimeters, whereas at the easternmost of our survey sites
the total depth of the stratigraphic sequence is nearly 3 meters. The Aguacate has generally been regarded as a culturally sterile substratum. However, during test excavations at
site G-175; a pitlike feature extending into the Aguacate was found; artifacts were also
found in situ in the top of the Aguacate at several survey sites. Evidence from both survey and excavation thus indicates that this assumption may be in need of reevaluation.
Topography of the area surrounding the Laguna de Arenal varies widely. There is
a general east-west trend, with steeply sloped to mountainous terrain in the eastern portion of the region grading into more gently rolling hills at lower elevations at the western
end of the lake. This general trend is subject to considerable local variation, however.
Prior to filling of the laguna, the Rio Arenal drained not only the ancient lake but also
a fairly wide and flat river valley with an extensive tributary system to the east. The Rio
Arenal forms part of the drainage system of the Rio San Carlos, which in turn is a major
tributary of the Rio San Juan. This river empties into the Atlantic Ocean and today
forms the political boundary between Nicaragua and'northeastern Costa Rica.
Although hydrologically part of the Atlantic Watershed, the Cuenca de Arenal is
a microcosm of the general east-west climatic gradient seen throughout northern Costa
Rica. Within the cuenca, there is a pronounced climatic gradient from the "Pacific re-'
gime" with more distinct seasons and lower rainfall in the west to the "Atlantic regime"
with increasing rainfall and less seasonality in the east. Local variations may be rather
dramatic, however. This variability is in part a reflection of the geographic location of
the cuenca in a tectonic depression between the Cordillera de Guanacaste and the Cordillera de Tilaran where the two climatic regimes meet. Local conditions such as elevation,
slope, temperature, exposure to seasonal winds from the southwest, force of the wind,
and solar radiation also contribute to this variability.
SURVEY STRATEGY AND METHODS
The survey was conducted from March 20 to April 4, 1984 and covered approximately 63 kilometers of the shoreline of the Laguna de Arenal, from the Quebrada Garrapa-
62
VINCULOS
ton on the northern shore, around the west end of the lake, and along the southern shore
to a point approximately 3 kilometers east of the mouth of the Rio Chiquito. This distance was traversed on foot by a team of 2 or 3 archaeologists and 2 or 3 local workers.
The area surveyed was essentially a transect of the sites discovered, the width of which
was determined by the distance from the shore of the lake to the eroded cut bank.
There were, of course, areas which were impassable on foot due to density of vegetation or marshy terrain. Such areas were usually encountered near the numerous quebradas which drain into the laguna. Other areas were in fact traversed but the subsurface had
not been exposed by erosion and, hence, it was not possible to determine whether or not
sites exist at these localitie~. Approximately 13 kilometers of the shoreline was so designated. Thus, the effective shoreline distance where the presence or absence of sites could
be determined was approximately 50 kilometers.
Survey methods were determined by first assessing the relative extent and density
of cultural materials. Cultural materials were then assigned to one of four classifications:
1) Large sites: relatively dense concentrations of artifacts (over 100), usually spread over
a fairly large area of ground surface, 2) Small sites: lesser concentrations of artifacts
(30-100) and covering a smaller area. These two categories have definable boundaries
within which there is a greater concentration of artifacts. 3) Sherd scatter: a light concentration of artifacts, usually less than 30 sherds, 4) Isolated finds (I F 's): an isolated
finished artifact or a very small number of diagnostic ceramics (less than 10) and any
associated cultural material.
Procedures for recording and making collections differed according to these classifications. For all sites the first step was to determine the extent of the site. UTM coordinates were then recorded and locations plotted on 1 :50,000 Tilaran or Arenal quadrangle maps.
Isolated finds were collected. Notes were taken, and Lot cards were filled out for
each IF.
Observations about sherd scatters were included in field notes, but no collections
were made. Although these sherd scatters often included some lithics, no scatters of predominantly lithic artifacts were found.
For all sites, a sketch map was drawn. Length and width of the site were determined
by pacing. The cut bank, or an eroded section of the shoreline, was cleaned and profiles measured and drawn to record stratigraphy. Observations of strata in both the cut
bank profile and the eroded section of the shoreline were compared with the regional
stratigraphic sequences in order to determine in which strata artifacts were embedded and
from which strata artifacts on the surface were most likely eroding. A representative
sample of fir'e-cracked rocks of various sizes was gathered, and total number of such
stones counted. Photos were taken and sites were recorded on site forms provided by the
Museo Nacional.
Collection methods differed for large and small sites. In both cases, special collecItions were made of any concentrations of artifacts and designated as Lots A-3 through
A-n. Examples of such collections would be a concentration of chipped stone, or of
sherds believed to belong to a single vessel, collections from profiles, and collections of
artifacts in situ.
On small sites, all artifacts observed on the surface were collected. This general
collection was designated Lot A-l, and included all diagnostic ceramics, all chipped stone
artifacts, and all groundstone artifacts.
- LAGUNA DE ARENAL. SHORELINE SURVEY
63
For large sites, areas of special collections were first designated. A 100 0 /0 sample,
Lot A-2, of a representative area of the site was then collected until a given volume of
artifacts was collected (half of a large bag); this collection was usually made as a 1 meter
wide transect of the site. Following this, the general surface collection was made. For
sites with an especially high density of artifacts, where it would have been infeasible to
collect all diagnostic ceramics, we selected a wide variety of diagnostic ceramics.
Data on a number of environmental variables which could be expected to influence
settlement patterns were also obtained. On-site measurements of the slope of the site
from the cut bank were made with a Brunton compass at representative locations.
Where topography changed rapidly, several measurements were taken. Field notes included information on topography, surrounding vegetation, and probable extent of the
site beyond the shoreline where possible. Additional information on landforms and
access to water was gathered from previous versions of the Tilaran and Arenal quadrangle maps, which show former river courses and the old lake shoreline.
Field assessments of stratigraphic levels were based on the Silencio and Tilaran stratigraphic sequences (Article 3). Knowledge of the regional stratigraphy based on observations .from both' excavations and road cuts was combined with on-site observations of
strata, and examination of soil and tephra samples with a microscope in the field laboratory, to establish stratigraphic correlations between sites and with the EI Tajo sequence.
The Silencio sequence is used for sites G-156 through G-161, and sites G-171
through G-l77. These sites are in the eastern half of the survey area, closer to Volcan
Arena!. Here Units 20 and 41 are distinctive and readily recognizable, allowing us to
distinguish between Unit 30 and the Unit 50 Complex. Presence or absence of artifacts in
either or both of these units could then be used to infer whether a site was a single component site or a multicomponent site. Finer stratigraphic divisions within the Unit 50
Complex defined in the Silencio sequence were seldom discernible along the eroded
shoreline.
The Tilaran sequence (Article 3) was used for stratigraphic designations at sites
G-162 through G-165, and G-167 through G-170, in the western half of the survey
area. At all of these sites erosion has exposed the Aguacate, but the complete sequence
is present at none of these sites. Hence, the identification of a particular Black or White
layer in the field was dependent on observation of dacitic pumice which is characteristic of White 3 and correlated with Unit 7 at EI Tajo (Article 3). This distincti.on is not
always possible in the field.
The entire Tilaran sequence predates Unit 41 in the Silencio sequence and is characteristic of a different mode of eruption for Volcan Arena!. Longer, less violent erui>tions of finer tephra probably typified this phase. It precedes the pyroclastic eruptions of
dacitic pumice typical of Arenal's more recent history (Melson, personal communication
1984).
RESULTS
Of the 22 sites recorded during the survey, 20 are located on the lakeshore. One
(G-166) is an island which was surveyed in the same manner as small sites on the shore
(except for transportation to the site!). Another, G-169, is located on a ridge above site
G-156; the owner of the property on which this site is located informed us of some pre-
64
VINCULOS
vious finds on his land and allowed us to conduct test excavations there with very promising results (Article 5).
No sites were found along the northern shore. However, the distance surveyed in
1984 was small and geological processes are not conducive to finding sites here. Because
the dominant winds from the northwest drive the waves of the lake to the southern shore,
eroded areas along the northern shore tend to be small and erosion shallower than along
the southern shore.
Fourteen sherd scatters were noted during the course of the survey, and it is possible that some of these actually are derived from small sites. However, no cultural materials were found in situ at any of these locations. Nineteen isolated finds were also found
and collected.
Site distributions and environmental factors
In attempting to reconstruct the prehistoric environment and potential resources of
the Cuenca de Arenal, the 512 meter lake level has been used as a guideline for possible
prehistoric topographic relationships. Locations of sites in relationship to this lake level,
former landforms and permanent water resources are shown in Figure 1.
The ecological relationships between climatic variability, undisturbed vegetation
and land use potential within the Cuenca de Arenal can be explored with reference to
data from meteorological stations and the Holdridge Life Zones, or "bioclimates." Each
life zone contains unique ecosystems with distinctive environmental conditions and associations of plants and animals. The primary vegetation in each bioclimate is determined
by the seasonal variation and distribution of temperature and rainfall (Hartshorn 1983).
Each life zone could thus be expected to have a different resource potential which would
have affected prehistoric adaptations. There are two major life zones within the survey
area.
Of the 20 shoreline sites, 17 are in the eastern half of the survey area. Thirteen are
located along a stretch of shoreline approximately 15 kilometers long between the Rio
Tronadora and Rio Chiquito. Of the 7 sites to the west of the Rio Tronadora, 4 are
within 15 kilometers of the Rio Tronadora and only 3 are located farther west. Sites
G-167 and G-168 at the far west end of the lake may actually be part of the same
site. They are separated by a large dike of boulders supporting the road around the lake.
For most of the sites on the shoreline (17 out of 20), the distance to water from
permanently flowing rivers or quebradas is less than the distance from the lake. Only one
site (G-170) -and of course the island (G-166)- is located closer to the lake than to a
perennial source of running water (See Table 1). Slopes throughout the area range from
7 to 18 degrees. I n only one instance (G-162) was the slope greater than 20 degrees.
The topography of the area between Rio Tronadora and Rio Chiquito, where the
greatest concentration of sites is located, is mountainous with numerous "quebradas"
cutting steep-sided hills of 700-900 meters in elevation, and forming ridges 500-600 meters in elevation as they reach the lakeshore. Sites G-171 through G-175 are located on
slopes about 20 meters below the crest of such ridges. Today these are points of land
exposed to erosion. Large sites in this area wrap around the points on which they are
located, indicating that the sites themselves probably include the ridge tops. The ridges
on which these sites are located overlook the alluvial plains of the Rios Arenal and Chiquito, or one of their tributaries. Sites G-176 and G-l77 are situated on the alluvial
terraces of the Rio Chiquito.
LAGUNA DE ARENAL. SHORELINE SURVEY
65
0'1
0'1
<
z
n
c
t:J
r
oVl
·tr
~
~CC,
C.,e"'lc
•
Ph •••
•
T, ....... , .
•
Ar .... '
6
Stt.ful.
•
Til., ...
~
1
k ..
~
~,
Figure 1. Location of sites discovered during the shoreline survey of Laguna de Arenal in
relationship to former lake level (512 m), landforms and permanently flowing 'AIOter.
The predominant ceramIc phase at each site is indicated.
Sites G-171 through G-l77 are in the Bosque muy humedo Premontano (tropical premontane wet forest) life zone. Original vegetation was probably a medium to tall
semievergreen forest with a few dry season deciduous species (Hartshorn 1983). Data
from meteorological stations indicate that rainfall ranges from 2800 mm annually at
Arenal Viejo, near Site G-174, to 3333 mm of annual precipitation at Rio Chiquito,
near sites G-176 and G-l77. At Arenal Viejo, there are 11 wet months and one month
of excessive precipitation, and no dry months. Moving east and south to Rio Chiquito,
there is one dry month, 5 wet months and 6 months of excessive moisture (Tosi 1980).
This life zone belongs to the perhumid province which is highly productive for
tropical forests, but has some limitations for agriculture. During months of excessive
rainfall, soils may become saturated, and anerobic soil conditions may develop, making
the soil difficult to work. During this period, open fields are subject to massive erosion,
especially on steep slopes. The soils are generally low in fertility and very acid, except in
terrain with low slopes or exceptional fertility, such as alluvial soils (Tosi 1980). All of
the sites in this zone have access to such alluvial soils. On the flood plain of the Rio
Chiquito, where sites G-176 and G-l77 are located, the land is subject to severe erosion
and occasional inundations, however. Crops that cannot tolerate a pronounced dry season are best adapted to this area, whereas seed crops such as maize which require a dry
season may be less well adapted to such a region.
The 10 sites between G-165 on the west and G-161 in the east are located on
points of land, hillsides or small coves today, but before the filling of the lake they would
have been in the area of transition between the steep hills to the south and broad flat
plains stretching north from the foothills to the lakeshore. G-160 and G-161 would
have been on large relatively flat benches. West of the Rio Tronadora, the terrain becomes
gradually less mountainous, although slopes in this region can be very steep. Four of
these sites (G-162 through G-165) are located in this hilly terrain north and west of
the RIO Tronadora.
The three sites at the western end of the lake are located on piedmont slopes. The
relief in this area is strongly undulating, but the elevation of the surrounding hills seldom
exceeds 700 meters. It is at th is end of the lake, where slopes are gentler and the depth
of the lake shallow, that the former lake was marshy (Tosi 1980).
All of the sites between the Rio Piedra at the west end of the lake and Quebrada
Tronadorcita in the east (site G-167 at the west end of the lake through site G-161 in
east) are in the Bosque humedo Tropical, Transicion a Premontano (tropical moist forest,
premontane belt transition) life zone. Sites G-166 and G-169 are also in this life lone.
The original vegetation of this life zone would have been tall, multistoried semideciduous
or evergreen forest (Hartshorn 1983).
The climatic gradient within the cuenca is apparent even within this single life lone.
Meteorological data from Rio Piedras and Naranjos Agrios, near sites G-167 and G-168,
show that this area has a dry season ~f 2 to 4 months, a wet season of 7 to 10 months,
and at most one month of excessive rainfall. In the same life zone but farther east, sites
near Tronadora receive' slightly more rainfall than the westernmost sites, but lack a real
dry season. This area is more affected by humid winds from the Atlantic. Eleven months
of wet weather and one of excessive precipitation have been recorded in this are~.
Annual precipitation is 2233 to 2440 mm at the western end of the lake, increasing to
2600 mm at Tronadora. Losses to runoff are low compared to total precipitation.
This is one of the life zones of the humid province. Life lones of this province
.
LAGUNA DE ARENAL. SHORELINE SURVEY
67
Table 1. Topographic, climatic, stratigraphic and artitaetual data by site
Distance to
Distance Permanent
to Lake Water
Life
Zone
9
1.1 km
Bh-T
G157 Foothills
G158 Foothills
G159 Foothills
G160 Bench
G161 Bench
1&
17
12-17
12
12
1.0 km
1.05 km
700 m
600 m
450 m
500
600
350
400
300
m
m
m
m
m
Bh-T
Bh-T
Bh-·T
Bh-T
Bh-T
G162 Foothills
G163 Foothills
G164 Foothills
23
7
7
700 m
750 m
350 m
450 m
750 m
350 m
Bh-T
Bh-T
Bh-T
G165
Foothills
12
300 m
200 m
Bh-T
G166
Island
nm
50 m
1.05 km
Bh-T
17
1.95 km
50 m
Bh-T
Site
Landfrom
G156
Foothills
G167 Piedmont
Slope
50 m
G168 Piedmont
12-15 1.7 m
200 m
Bh-T
G169 Ridge
G170 Piedmont
G171 Ridge
Gl72 Ridge
Gl73 Ridge
G174 Ridge
G175 Ridge
G176 Alluvial terrace
Gl77 Alluvial terrace
nm
12
17
18
12
13-17
10-14
10
10-13
50 m
450 m
150 m
300 m
400m
450 m
50 m
200 m
50 m
Bh-T
Bh-T
Bmh-P
Bmh-P
Bmh-P
Bmh-P
Bmh-P
Bmh-P
Bmh-P
Note:
68
600 m
200 m
550 m
250 m
400 m
1.7 km
2.1 km
3.1 km
4.25 km
Strata
Unit 30
Unit 50
Unit 30
Unit 50
Unit 30
Unit 30
Unit 30
Unit 50
1st Black
1st Black
1st Black
White 2
Black 2
White 3
1st Black
White 2
Black 2
Unit 30
Unit 50
Top of
Aguacate
1st Black
Top of
Aguacate
White 1
Black 2
na
1st Black
Unit 30
1st Black
Unit 30
Unit 30
Unit 50
Unit 30
Unit 30
Unit 40
Unit 50
Permanent water refers to permanently flowing rivers or quebradas
Bh-T = Bosque humedo Tropical, Transici6n a Premontano
Bmh-P = Bosque muy humedo Premontano
Strata refers to strata in which artifacts in situ were observed.
VINCULOS
Dominant
Ceramics
Arenal
Silencio
Arenal
Arenal
Tilaran
Tilaran
Arenal
Tronadora
Arenal
Arenal
Arenal
Arenal
Arenal
Tilaran
Arenal
Arenal
Arenal
Tilaran
Silencio
Arenal
Arenal
Arenal
offer almost ideal conditions for agriculture, with sufficient soil moisture even in the dry
season. Compared to the more humid bioclimates, such as the Bosque muy humedo Premontana life zone in which sites G-171 through G-l77 are located, the soils are relaiively fertile, less acid and higher productivity can be maintained. The soils, long hours of
sunshine, and relatively low humidity make the region well-suited for cultigens with a
short growing season. Although suitable for cultivation, the land on which sites near
Tronadora or farther east are located is more favorable for permanent or semipermanent
cultigens (Tosi 1980). When cultivated using methods which do not involve a great deal
of soil reworking, maize and beans can be considered semipermanent crops (Tosi 1980).
Most Precolumbian agricultural" practices, such as swidden agriculture, could probably be
placed in this category. However, the practice of burning fields before planting, typical of
maize agriculture in much of the New World, subjects the soils of this area to potentially
severe erosion (Tosi 1980).
The erosional environment of the s.urvey area can result in problems as well as advantages for the archaeologist. The areal extent of sites cannot be determ ined from a
shoreline transect. The proportion of a site that had been inundated was impossible to
calculate and, except at those sites selected for test excavations, we have not yet attempted to define the extent of the sites beyond the shoreline. Relative comparisons of site
sizes made on the basis of the maximum extent of cultural materials along the eroded
shoreline thus may not be representative of actual site sizes. We were therefore unable
to establish any sort of site hierarchy. Nor can we totally eliminate the possibility that
the estimated size of a site is, at least in part, a function of geological processes.
Architecture is lacking and features, if they survive, can be difficult to recognize in
an erosional environment. In fact, no definitely cultural features were recorded during
the survey. The only clues as to whether or not a site was a habitation site were rather
scanty surface collections of groundstone artifacts such as manos and metates, chipped
stone, and domestic ceramics.
Manos or metates were collected at several sites. All showed evidence of use (Article 11), but whether in a domestic or ceremonial context cannot be determined. Groundstone celts, or hachas, were also found at a numer of survey sites. These tools are frequently interpreted as indicating woodworking activities such as forest clearing at habitation sites. Combining this information with that from lithic analysis of chipped stone
(Article 10), available data indicate that three sites have both groundstone and chipped
stone artifacts that are usually associated with domestic activities. These sites are G-156,
G-161, and G-163. These were among the sites selected for test excavations. Data
from ceramic assemblages which might support the supposition that these are habitation
sites are not yet available.
Two sites, G-166 and G-164, have been identified as cemeteries. Both may have
habitation areas as well.
Ceramic chronology and stratigraphic correlations
Middle Formative Tronadora Phase ceramics represent the earliest settlements in
the Cuenca de Arenal, and are dated from 1000 to 500 B.C. This phase comprises the
major ceramic component only at Site G-163. They are almost as abundant as Arenal
Phase ceramics at Site G-162, however (with 41 percent Arenal Phase ceramics and 36
percent Tronadora).
LAGUNA DE ARENAL. SHORELINE SURVEY
69
eo
eo
•. 19
·•
; eo
u 10
•
.. 00
20
10
I~ I ...
'
,"
'"
:
170
11
11.
tI
".
.
,.3
,.z
L ,..
"1
,"
"~I
15'
, •• ,"
~ .J
"0
,"
I~
I
t71
172
,,3
,.,.
11'
~
17'
I
1
111
Figure 2. Percentages of ceramic phases at lakeshore sites. Site numbers are arranged
geographically, from west to east.
At 14 of the 20 shoreline sites, and at the island site G-166, Arenal Phase ceramics,
dated from 500 B.C. to 500 A.D., are the major component, and percentages of this phase
in the individual site assemblages tend to be high (Fig. 2). At Site G-156, ceramics from
excavated and survey lots combined show equal percentages of both Arenal and Silencio
ceramics. However, ceramics from escavated lots are predominantly Arenal Phase and
no Silencio Phase ceramics were recovered from excavated units.
Locations of Arenal Phase sites appear to be related to topography. The three sites
in the western half of the survey area are all Arenal Phase sites; all are Inctted on wide
piedmont slopes. Sites G-167 and G-168 are farther from the old lakeshore than any
other sites. This may be related to the marshiness of the edge of the Idke at this end.
Twelve of the 15 Arenal Phase sites are located in the eastern half of the survey area,
and 8 of these 12 are in the "intermediate" hilly zone to the west of Quebrada Tronadorcita. There does not seem to be any relationship between the life zones and site locations, although access to resources from the higher altitudes just south of these sites may
have been a factor. Such resources might have included wild game and tree and root
crops.
Arenal Phase sites in the eastern part of the survey area, where the Silencio stratigraphic sequence is clearest, were observed to have artifacts in situ in the Unit 50 Complex. At sites where the Tilaran stratigraphic sequence was used, artifacts were found in
the top of the Aguacate, Black 1, White 2, and/or Black 2. Arenal Phase ceramics predominated in only three cases where artifacts were observed eroding from Unit 30. The
presence of early ceramics in more recent strata could be explained by a number of factors. One of these sites (G-159) has been greatly disturbed by construction activities;
disturbance or reworking of soils at other sites through cultural activities or natural pro-
70
VINCULOS
ccsscs such as rodent disturbance may also have occurred. The Arenal Phase spans a long
time and it is possible that either Stratigraphic associations or ceramics were misdiagnosed.
Sites at which Silencio (600 to 1000 A.D.) or Tilaran (1000 to 1500 A.D.) Phase
ceramics are the major components are conspicuously fewer. Most of these sites are located between the Rio Tronadora and Quebrada Tronadorcita. Silencio Phase ceramics are
primary only at sites G-174 and G-157. At only three sites along the lakeshore are
Tilaran Phase ceramics the primary component. Generally, Silencio and Tilaran Phase
ceramics have been found as major and secondary components at the same sites. Only at
G-173 and G-166 are Arenal Phase and Tilaran Phase ceramics the dominant major
components, with very low percentages of the intervening Silencio Phase ceramics. The
G-169 ceramic assemblage contained 39 percent Tilaran and 28 percent Silencio ceramics.
Stratigraphic observations made at Silencio and Tilaran Phase sites consistently indicated the presence of artifacts in Unit 30.
Test Excavations
Several sites located by the survey were selected for test excavations during the
1984 field season. Sites G-156 and G-161 were chosen because surface collections and
stratigraphic observations indicated that they might be multicomponent habitation sites.
G-163 was selected as a probable single component Formative site, and thus far is the
only site at which Tronadora Phase ceramics are dominant. It also seems to be a habitation site. A projectile point probably dating to the Archaic period was also found at this
site (Article 10). G-175 was tested to examine the hypothesis that it is a multicomponent site with a Formative component and a Zoned Bichrome/Early Polychrome transitional phase. Posthole survey of site G-169 revealed a high density of ceramics and a
likely habitation site. Details of these investigations are presented in Article 5 of this
volume.
Several other sites of interest have yet to be test excavated. One of these is site
G-164. This site had an exceptionally high artifact density. On the hill above is a prehistoric cemetery. The extent of the cemetery has not yet been determined. The ceramic assemblage showed tremendous variety, including fine-paste wares and types similar
to Atlantic styles or trade wares, as might be expected at funerary sites (Snarskis, personal communication 1984), and domestic wares (Hoopes, personal communication
1984). A fragment of a serpentinite pendant found at this site also implies external
contacts, either by trade or direct procurement, for the Cuenca de Arenal (Article 12).
The site includes several lithic concentrations. In situ cultural materials can be found in
the five lowest stratigraphic levels.
Site G-170 was not actually found during the course of the survey, but about 2
weeks later during a geological reconnaissance, at which time the lake level was considerably lower. This is a large site at the west end of the lake, and appears to be a single
component site of the Arenal Phase. All materials found in situ came from the first
Black soil above the Aguacate.
DISCUSSION AND SUMMARY
The earliest settlement in the Cuenca de Arenal for which we have evidence is Late
Period IV, Tronadora Phase (1000-500 B.C.). However, the surface find of a probable
·LAGUNA DE ARENAL. SHORELINE SURVEY
71
Archaic point at Site G-163 raises the possibility of an even earlier occupation.
Ceramic evidence points to a fairly heavy population on the slopes around the lake
during the Arenal Phase, which corresponds to Late Period IV (300 B.C. to 500 A.D.).
This hypothesis is supported by data from previous surveys in the Cuenca de Arena!.
Murray found Zoned Bichrome sites along the shore at the 512 elevation level at the
western end of the lake (Article 1). Several sites within the now-inundated part of the
Rio Arenal valley, including EI Tajo, also were designated as Zoned'Bichrome sites (Aguilar 1984). Two other surveys have been conducted in the highlands of northwestern Costa Rica. Survey data from the Rio NaranjojBijagua Valley in the Cordillera de Guanacaste, an area similar to the Cuenca de Arenal in terms of both topography and climate, indicated that population levels were highest during the Zoned Bichrome period, although
the area was occupied earlier (Norr 1982). Sites at Hacienda Jerico, on the western slopes
of the Cordillera de Guanacaste, were also dated to this period and the Early Polychrome
(Finch 1982).
The role which the resources of the lake itself or volcanic processes contributing to
soil fertility may have played in these developments has not yet been fully explored.
Tectonic depressions, especially when related to water resources or trade routes, may
well have been im portant factors in settlement patterns and population density throughout
Precolumbian lower Central America (Lange 1984b). Both are possibilities for the Cuenca de Arena!.
The increase in population during Late Period IV parallels developments in other
areas of Costa Rica. In the Greater Nicoya subarea, population increased and mortuary
data indicate social stratification and at least low level chiefdoms. Subsistence data is
rare, however, until the later part of this period. At that time there was a marked shift
to coastal adaptations and exploitation of marine resources (Lange 1984b).
On the
Atlantic Watershed, this was also a time of increased population growth 'and sites from
this period are numerous and large, usually situated on fertile plains or valley floors. As
in Guanacaste, a ranked social structure is indicated. Direct evidence for maize cultivation has been obtained from several sites (Snarskis 1981 a, 1984). Wh ile our evidence of
increased population levels during Late Period IV accords well with that from others
areas, the very limited data available on subsistence (Article 13) and social structure
precludes comparison with other cultural and social developments at this time.
Population of the area seems to have declined dramatically during early Period V
(600 A.D. to 1000 A.D.) and remained low through late Period VI (1000 A.D. to 1500
A.D.). Whether this phenomenon is related to volcanic activity has not yet been investigated. A similar decrease in population also seems to have occurred in the Rio Naranjo/Bijagua Valley, with some sites approaching abandonment (Norr 1982).
This contrasts with general developments in other areas of Costa Rica. In the
Greater Nicoya, population growth continued and Middle Polychrome sites are numerous;
sites continue to be concentrated along coastal embayments through PeriOd VI (Lange
1984b). In the Atlantic Watershed, dispersed villages located on alluvial terraces continue during Period V (Snarskis 1981), but again there seem to be fewer sites (Snarskis
1984). During Period VI there are numerous but relatively small, agglomerated villages
located on the fertile plains and valleys of eastern central Costa Rica (Snarskis 1984).
With only very limited data available on site locations before the construction of
the Sangregado dam, and lacking data on wider regional patterns, determining critical factors in settlement patterns is difficult. We can only address the question of settlements
72
VINCLLOS
within the cuenca itself at the 540 meter elevation. Here, important factors in the selection of settlement locations seem to have been access to rivers and flat land, although
access to resources of the adjacent highlands may also have been a factor. The majority
of sites found by Aguilar (1984) were located near tributaries of the Rio Arena!. This
seems to agree with the general pattern implied by our data.
At first glance it may seem that in terms of climate and topography the greatest
concentration of sites is located in the least favorable area. In terms of agricultural potential, sites in the eastern sector are in an area limited by both climatic and topographic
factors, whereas those at the western end of the lake are favorable in climate and limited
mostly by local topographic factors. However, the easternmost sites have access to the
alluvial plains below and because of their elevated locations probably were not subject
to periodic inundation. The distribution of sites at the west end of the lake may also
be somewhat misleading. It indicates only the distribution of sites at the 540 meter lakeshore. If the distribution of sites along the old lakeshore were known, it would be possible to present a more realistic view of prehistoric occupation of this area.
LAGUNA DE ARENAL. SHORELINE SURVEY
73